Armed with a handgun, the 42-year-old ex-convict moved methodically from victim to victim, placing his handgun against their head or neck and then pulling the trigger, authorities said.

The unsuspecting victims worked just feet apart in the deboning room and the grinding room at Valley Protein, but they wore ear protectors and the gunshots were drowned out by the machinery.

Jones tried to fire at a fifth person, police said, but ran out of bullets.

Following the shootings, Jones walked out of the building and unloaded the empty casings. He then reloaded, placed the gun against his head and fired. He was pronounced dead at a hospital a few hours later.

Jones' motive was unknown, but it was clear he had targeted his victims, Fresno Police Chief Jerry Dyer said. About 30 employees were working at the plant during the shooting.

"He had opportunity to shoot other co-workers that were in the business at this time, but he chose not to," Dyer said. "He walked around them in order to get very close to the intended targets, place the gun very close and fire a round."

Some workers told police Jones did not appear to be himself when he arrived for his shift.

"We are still trying to follow up on some rumors regarding a dispute between Jones and one of the other co-workers, but we have not been able to verify it at this point," Dyer said.

Jones, who had worked at the plant for 14 months, had an extensive criminal history, authorities said. In 1994, he was sentenced to 13 years in prison for robbery, then released on parole in 2001, according to the California Department of Corrections and Rehabilitation.

Jones also served time for robbery and later for vehicle theft and other convictions, and was paroled from prison on June 1, 2011, department spokesman Luis Patino said. Jones was discharged from parole on May 1, 2012, Patino said.

"He was reviewed and he had not had any violations and so he was discharged from parole," Patino said.

Dyer said Jones arrived at work on a bike just before 5 a.m. About three-and-a-half hours into his shift, he pulled out the handgun.

First, Jones silently walked up to 32-year-old Salvador Diaz in the grinding room, put the handgun to the side of his head and pulled the trigger, police said. Diaz was pronounced dead at the scene.

Jones then went next door to the deboning room, put the gun up against the head of the second victim, 34-year-old Manuel Verdin, and pulled the trigger. Verdin was pronounced dead at a hospital.

Jones shot the third victim, Arnulfo Conrriguez, 28, in the neck at close range, police said. Conrriguez was in serious condition at Fresno's Community Regional Medical Center, hospital spokeswoman Mary Lisa Russell said.

A fourth victim, 32-year-old Fatima Lopez, witnessed the slayings and started to run. Jones shot at her, striking her in the buttocks. Lopez was treated at Community Regional Medical Center and released, Russell said.

Attempts by The Associated Press to reach Lopez for comment were not immediately successful.

Plant worker Estevan Catano, 21, escaped injury when the gun Jones placed against his head did not discharge because it was out of ammunition, police said. The gun only held four rounds.

Investigators secured Jones' Fresno apartment, where they found 24 rounds of .357-caliber ammunition -- the type used in the shooting -- and 21 rounds of .38-caliber ammunition.

Police said the handgun used by Jones -- a .357 Derringer -- is an expensive and rare weapon and they are trying to determine whether it was stolen. The serial numbers on the gun were filed off, Dyer said.

Valley Protein, where Jones worked for more than a year, was established in 2005, according to online business records. A call to the company went to a voicemail recording that said, "due to an emergency, we are closed for the day."

A woman who answered the phone at a listing for the company's CEO, Durbin Breckenridge, and identified herself as his wife said she would pass a message to him.

News media and onlookers were kept several blocks from the plant in the morning, as police used yellow tape to block access. Dozens of officers swarmed the area.

Joe Martinez, 45, told the Fresno Bee that he was in the drive-thru lane of a fast-food restaurant when he heard a loud pop that he initially thought was a car backfiring.

Then he looked to the north and saw a man on the ground with two people standing over him.

"It's the last thing you expect to see," the newspaper quoted Martinez as saying. "It's very upsetting."